1. Field of the Invention
The present invention relates to a method of fabricating a photomask blank, and more specifically, to a technique of fabricating a photomask blank as a material for a photomask used to micromachine semiconductor integrated circuits, CCDs (Charged-Coupled Devices), color filters for LCD (Liquid Crystal Display) devices, magnetic heads, and the like.
2. Description of the Related Art
Lithography techniques are used to, for example, fabricate semiconductor integrated circuits, which have been increasingly highly integrated. For the lithography techniques, efforts have been made to reduce the wavelength of exposure light used for light exposure apparatuses in order to improve resolution. According to a road map for lithography updated by ITRS (International Technology for Semiconductors) in 2004, the reduction in wavelength has progressed from a g line (wavelength λ=436 nm) and an i line (λ=365 nm) which are ultraviolet light sources to a KrF line (λ=248 nm) and an ArF line (λ=193 nm) which are far ultraviolet light sources. Further, the technology is expected to shift to ArF water immersion for a half pitch of 65 nm, hp 65, in 2007 and to a combination of F2 or ArF water immersion and a resolution enhancement technology (RET) for a half pitch of 45 nm, hp45, in 2010.
Thus, demands for photomasks (and photomask blanks as materials for the photomasks) at the forefront of the technology are expected to be ensured at least until 2010. It is also pointed out that the lithography with photomasks is likely to be used for a half pitch of 32 nm, hp32, which is expected to be introduced in about 2013 and for a half pitch of 22 nm, hp22, which is expected to be introduced in about 2016.
According to the Rayleigh's equation, corresponding to an evaluation amount for resolution, resolution pitch RP and the depth of focus DOF are given by Formulae (1) and (2) using proportionality factors k1 and k2, respectively. Consequently, for miniaturization for the lithography technology, not only the reduced wavelength as described above but also an increased numerical aperture (NA) is required.RP=k1λ/NA  (1)DOF=k2λ/NA2  (2)
The “water immersion technique” has been gathering much attention as a technique for increasing the NA. The water immersion technique increases the NA value by filling a liquid of a refractive index (n) higher than that of an atmosphere (gas) in an exposure environment, between a wafer to be exposed and a lens provided closest to the wafer to set the NA value as refractive-index-value times (n times) as large as that of the liquid.
That is, if the spread of a light beam formed into an image at one point on the wafer to be exposed is defined as ±θ, NA=n0·sin θ where nodenotes the refractive index of the wafer side. However, since the wafer side is usually air (n0=1), NA=sin θ. Consequently, filling the liquid of the refractive index n between the wafer to be exposed and the lens results in NA=n·sin θ, enabling an increase in numerical aperture NA. This in turn enables a reduction in resolution pitch RP.
As is apparent from Formula (1), shown above, a reduction in k1 factor is also an effective method for reducing the resolution pitch RP. RET for this purpose is based on “modified illumination” in which the shape of an effective light source, which is a simple circle, is modified, “multiple exposure” such as FLEX in which the wafer is exposed by being moved in the direction of the optical axis of a projective optical system using the same mask, or the like.
On the other hand, as is apparent from Formula (2), shown above, the reduction in exposure wavelength is effective for a reduction in resolution pitch RP but reduces the depth of focus DOF, affecting manufacturing yield. That is, in spite of the advantage of reducing the k factor to allow microstructures to be transferred, the reduction in exposure wavelength reduces the depth of focus DOF, resulting in a focus error and thus a decrease in manufacturing yield if the photomask is not sufficiently flat.
One method for solving this problem is phase shifting method. The phase shifting method uses a phase shift mask to form patterns such that the phases of adjacent patterns differ by about 180°. Specifically, a phase shift film provided in the phase shift mask changes the phase of exposure light by 180°. Consequently, light having passed through an area in which the phase shift film pattern is formed and light having passed through an area in which the phase shift film is not present have a light intensity of 0 at the boundary between the areas. This leads to a light intensity distribution indicating a rapid change in that area. As a result, a high DOF can be obtained, improving image contrast. The phase shift mask includes a Levenson type and a halftone type. In particular, using the halftone phase shift mask drastically improves the DOF.
For the halftone phase shift mask, a single layer mask having a relatively simple structure has been proposed. Proposed single layer phase shift masks have a phase shift film consisting of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxinitride (MoSiON). For more information, see, for example, Japanese Patent Laid-Open No. 7-140635 (Patent Document 1).
To fabricate such a phase shift mask, a method is used which forms patterns for a phase shift mask blank by the lithography method. The lithography method involves applying a resist on a phase shift mask blank, exposing a desired portion to an electron beam or an ultraviolet ray, and then developing the phase shift mask blank to expose the surface of the phase shift film which has been exposed to the electron beam or ultraviolet ray. The exposed phase shift film is etched away through the patterned resist film as a mask to expose a substrate surface. The resist film is subsequently stripped to obtain a phase shift mask.
When a plurality of masks are used to form a multilayer structure for a device, a high overlapping efficiency is required. Increasingly miniaturized patterns further increase the required overlapping efficiency.
However, if stress is already accumulated in the thin film formed on the substrate which is in the form of a photomask blank, then while patterns are being drawn on the blank through the steps of resist application, exposure, development, etching, and resist stripping, the stress accumulated in the film is partly released. The resulting photomask is thus distorted. The distortion may reduce the overlapping accuracy of the photomask, causing defects in a circuit pattern to be drawn.
The level of the “distortion” depends on the pattern to be drawn and the magnitude of the stress accumulated in the film. It is very difficult to control or release the stress during the process of fabricating a photomask.
This problem can be avoided by forming thin films under conditions in which the stress of each thin film is almost zeroed. However, it is very difficult and virtually impossible to find fabrication process conditions in which film formation conditions for providing appropriate properties for thin films used as optical films also serve as conditions for forming thin films with reduced stress. It is thus necessary to have a step of forming thin films under conditions in which the appropriate properties of the thin films can be obtained and a separate step of reducing the stress of the thin films.
The thin films in the photomask blank such as the phase shift mask are generally formed by sputtering. However, stress may occur in the films during the film formation process and distort the substrate itself, warping the photomask blank. To solve this problem, a technique has been proposed which involves irradiating a light-absorbing thin film such as the phase shift film with light from a flash lamp at a predetermined energy density to control the film stress so that the distortion of the photomask blank is reduced. For more information on this technique, see, for example, Japanese Patent Laid-Open No. 2004-0223 (Patent Document 2).
Possible external energy applying means for reducing the stress of thin films include a hot plate, a heater, a halogen lamp, an infrared lamp, a furnace, and RTA (Rapid Thermal Anneal). However, owing to excessive energy application, resulting in an increase in substrate temperature, these techniques may damage the substrate itself or increase the time required for treatment. This disadvantageously degrades productivity. Consequently, light irradiation using a flash lamp such as the one described in Japanese Patent Laid-Open No. 2004-0223 (Patent Document 2) is excellent.
However, light irradiation using a flash lamp may increase the amount of light applied to an outer peripheral area of an optical film formed on the substrate above the amount of light applied to a central area due to reflection from a susceptor holding the substrate by a back surface of the substrate. This unfortunately causes an in-plane variation in the optical properties of the films.